Since 2.9′s release a lot of users have been complaining that they were unable to use the scheduled posts feature. While you could schedule the post, it would not publish at the time you wanted. In fact, you would have to go in and adjust manually publish.

Other problems that have been addressed can be found at the WordPress trac. Please note that if you are having actual upgrade issues to be specific about your problem or it will be difficult to resolve. If it is just plugin issues, either contact the plugin creator, or go to the WordPress Support Forums and post a new topic. Please note, some plugin creators may not provide support or request you to join their own forum.

The update should be available as an automatic download or for those still having trouble, you can download and manually upgrade your WordPress installation.

WordPress 2.9, also named Carmen, was announced on the WordPress development blog in their post, WordPress 2.9, oh so fine for anyone to download. Of couse, those who can automatically upgrade, you should be able to see the notice in your WordPress administration panel and be able to upgrade from there.

Some of the great features introduce were:
- Ability to mass upgrade plugins
- Post/ File Restoration for accidental deletions
- Built-in image editor
- Easier ability to embed videos
- Upgrading TinyMCE and SinglePie
- Support for canonical URLs
- Ability to register post thumbnails
- Improvement of PressThis
- Theme Editor Improvements
- Hooks and Filter improvements

In fact, there were quite a few mentioned at the development blog. Of course, this makes 1 step closer to merging WordPress and WordPressMU, which were hinted about earlier this year.